1. Field of the Invention
The present invention relates to a coil for an antenna and an antenna system.
2. Description of the Related Art
Conventionally, a technology has been known that uses an electrically-heated defogger as an antenna or a part of an antenna formed in a window glass for a vehicle, including multiple heater wires and bus bars connected to terminal parts of the wires to feed power.
In general, when a defogger is used as an antenna, coils are connected between a bus bar and a power supply, and between the bus bar and ground, respectively, to allow a direct current flowing, although a signal in a frequency band to be received by the defogger needs to be blocked.
However, since a relatively high current flows through the defogger, such as several amperes to several dozen amperes, it is necessary to provide a coil having a high current capacity that uses a thick conducting wire, and a problem has arisen in that the coil becomes larger as a whole, and has a greater weight.
For example, Patent Document 1 (Japanese Laid-open Patent Publication No. 2007-67171) discloses a coil that is downsized compared to a conventional coil having a single winding, by having the conductor wound by double windings. Further, this make it possible to shorten the length of a ferrite core to be inserted in the coil, and hence, to reduce the weight.
However, if using a coil having double windings as in the cited reference 1, the inner winding and the outer winding come close to each other in a part corresponding to the double windings. Therefore, capacitive coupling is formed between the inner winding and the outer winding, and this results in decreasing the impedance in a desired frequency band of the coil. Thus, a problem has arisen for simplistic use of a double-winding coil, in that a sufficient blocking performance is not obtained for a signal in a desired frequency band to be received by the defogger.
Thereupon, the present invention provides a coil for an antenna and an antenna system in which the size of the coil is small, and high impedance can be obtained in a desired frequency band.